Expansion tank with improved single diaphragm

ABSTRACT

An improved expansion tank is provided for at least temporarily storing a pumped liquid under pressure, the expansion tank comprising a thin walled outer shell formed of two substantially hemispherical domes joined together, either directly or at the two ends of a central, substantially cylindrical section, and a flexible diaphragm located internally of the tank and secured to the inner surface of the shell of the tank to divide the internal volume of the tank into a fluid-tight section holding a gas under pressure and a fluid-tight section for holding an aqueous liquid under pressure. The improvement comprising diaphragm coupling ring formed as a discrete part of the diaphragm tank that can be fabricated independent of the domes and any cylinder. The diaphragm coupling ring provides a robust, leak-proof seal with the diaphragm, so that it can be connected onto an inner surface of an expansion tank in a separate, secondary operation.

BACKGROUND OF THE INVENTION

Expansion tanks are known for use in flow systems for controlling flow of liquid under varying pressures. Most commonly, expansion tanks comprise a substantially cylindrical housing terminated on each end by a substantially hemispherical dome section. The entire combined tank being most preferably suitable for isotensoidal reinforcement by wrapping with equally stressed filaments. In some cases, the cylindrical housing may be shortened or absent, such that the entire shape is spherical, comprised of the two domes. The housing and domes further contain a bladder-type diaphragm that divides areas of a liquid and a pressurized gas. For a general discussion of expansion tanks and bladder-type diaphragms, see U.S. Pat. No. 4,784,181 to Hilverdink entitled “Expansion Tank with a Bladder-Type Diaphragm”.

In expansion tanks, it is critical to maintain a liquid and gas-tight barrier between the liquid and pressurized gas, as well as the outside environment. Any leakage between gas and liquid, or gas and outside, will cause the tank to stop working until it is recharged and may also cause permanent damage to the tank. This gas tight barrier must also be capable of flexing and bending, while maintaining its integrity through continuous changes in temperature and pressure, making material selection and seal joint design an integral part of overall tank perfoiniance.

Two general approaches to making this barrier have traditionally been used. For example, a first approach, as described in, among others, U.S. Pat. No. 7,322,488, and 7,303,091, “Expansion tank with double diaphragm”, includes a “double diaphragm bladder” secured to the interior of a tank. The bladder comprises a non-flexible diaphragm having a peripheral edge and a flexible diaphragm having a peripheral edge. The peripheral edges of the non-flexible diaphragm and the flexible diaphragm are sealed together with a ring clamp, or by heat sealing. This provides an excellent leak-proof seal. Most important in this design, the movement of the diaphragm in operation is decoupled from the outer, cylindrical housing and domes. Therefore, when the pressure differential between the water and air sections of the tank changes, and the diaphragm moves or is stretched, it does not pull on the walls of the cylinder. This approach, however has the disadvantage that it uses additional parts, including the large non-flexible diaphragm, along with corresponding additional fabrication steps, which adds both materials and manufacturing costs when compared to the second approach.

The second approach to the air-water barrier that is generally used is described in U.S. Pat. No. 7,671,754 Sensor for detecting leakage of a liquid; U.S. Pat. No. 5,368,073 Hydro pneumatic Pressure Vessel Having an Improved Diaphragm Assembly; U.S. Pat. No. 5,484,079 Hydro pneumatic Filament Wound Pressure Vessel; and U.S. Pat. No. 7,216,673 Non Metallic Expansion Tank With Internal Diaphragm and Clamping Device for Same. In this design, the diaphragm is directly coupled to the outer wall of the dome or cylindrical housing by either adhesive bonding or a mechanical clamping mechanism. While this second approach has a reduced number of parts compared to the first approach that was described, attaching the diaphragm directly to the wall of the tank is a fundamentally flawed design: as the pressure differential between the water and air sections of the tank changes and the diaphragm moves and stretches, the diaphragm pulls on the attachment point to the vessel wall. It is well known by those skilled in the art that thin-walled, large diameter cylinders and spheres are very poor in collapse conditions; by pulling inwards on the wall of the tank, it is possible to collapse portions of the tank construction. Just as importantly, it is well known by those skilled in the art that the bond strength between the dissimilar materials of construction of the tank can be very low; the force exerted by the diaphragm on the tank can cause delamination between different layers, such as the diaphragm (which is, typically, an elastomer or flexible thermoplastic), the outer wall (which is, typically, a rigid thermoplastic shell), or the fiber reinforcement (which is, typically, fiberglass in a thermoset). It can also cause interlaminar failure of the fiber reinforcement itself So, by coupling the diaphragm directly to the wall, permanent, catastrophic failure of the tank can result.

SUMMARY OF THE INVENTION

In this invention, expansion tanks with two novel improvements to the diaphragm seal are disclosed. Both improvements allow for robust seals to be fabricated with improved processing and manufacturing flexibility.

The first improvement is a diaphragm coupling ring. The diaphragm coupling ring is a discrete part of the diaphragm tank that can be fabricated independent of the domes and cylinder. The diaphragm coupling ring is specifically designed to provide a robust, leak-proof seal with the diaphragm, and then be connected to the expansion tank in a separate, secondary operation. The coupling ring being an independent part of the construction, it can be initially joined with the diaphragm using conditions, equipment, and processes that are not compatible with existing expansion tank manufacturing, before being joined into the tank.

The second improvement is the novel application of “tie-layers” to join the dissimilar materials of the diaphragm and the tank sections, e.g., the domes or the cylinder, the diaphragm and the coupling ring. Known processes to attach diaphragms to tanks only produce adhesion between the diaphragm and the surface layers of the tank wall; these joints are highly susceptible to adhesive failures under the operating conditions of expansion tanks. But with unique combinations of tie-layers with suitable manufacturing steps, the inventors disclose a process to produce a cohesive, covalent-, chemically- and/or theinially-bonded diaphragm that is ideally suited for the operating conditions of expansion tanks.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic cross-section diagram of a diaphragm tank according to an embodiment of the coupling ring invention, representing the tank charged with air pressure, but not with water;

FIG. 1B is an expanded schematic cross-section view of a portion of the diaphragm tank and coupling ring of FIG. 1;

FIG. 1C is an expanded schematic cross-section view of a portion of the diaphragm tank and the coupling ring of FIG. 1, plus a crimp ring inside of the coupling ring;

FIG. 2 is a schematic cross-section diagram of a diaphragm tank according to an embodiment of the tie-layer invention representing the tank charged with air pressure but not water pressure; and

FIG. 2B is an expanded schematic cross-section view of a portion of the diaphragm tank and tie-layer of FIG. 2.

FIG. 3 is a schematic cross-section diagram of a diaphragm tank according to another embodiment of the tie-layer invention, showing a polymer encapsulated crimp ring welded to the cylinder; and

FIG. 3B-B is an expanded cross section of the crimp ring and cylinder in FIG. 3.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

FIG. 1 is a cross section of an embodiment of a tank in accordance with this invention, with a single diaphragm, connected to a tank inner wall by a circumferential coupling ring 8; the tank is charged with pressurized gas, such as air, but empty of liquid, e.g., water. The tank comprises a substantially cylindrical housing section 1, joined to substantially hemispherical domes 5 and 6, with a single diaphragm 7 that is connected to an inside wall surface of the lower dome 6, adjacent the joint with the central cylindrical section 1. The peripheral edge 4 of the diaphragm 7 is circumferentially connected to the diaphragm coupling ring 8. Dome 5 further comprises an air valve 9, which allows the upper volume of the tank to be charged with air or an inert gas. Dome 6 further comprises a threaded connection 10, through which water can flow into the lower volume of the tank, under the diaphragm 7.

FIG. 1B shows a partial expanded view of a tank wall near the coupling ring 8. FIG. 1C shows an expanded view near coupling ring 8, that also comprises a metal crimp ring 11, holding the peripheral edge 4, of the diaphragm 7 in the mouth of the coupling ring 8.

In certain embodiments, the tank segments 1, 5 and 6, and the coupling ring 8, can be independently or together formed of substantially similar materials; these materials can be selected from a group of non-metallic materials including thermoplastics, thermosets, polymers, plastics, elastomers, rubbers, or multilayer materials comprising the same.

In certain embodiments the tank segments 1, 5, and 6, and the coupling ring 8 may be formed of polymeric materials selected from a group of thermoplastics including polyolefins, polyethylene, polypropylene, polybutylene, polyamides, nylon, PVC, CPVC, ionomers, fluoropolymers, or copolymers or multilayer structures comprising the same. The tank segments 1, 5, and 6, and the coupling ring 8 may also be formed of crosslinked polyolefins such as crosslinked polyethylene (PEX, PEX-a. PEX-b, PEX-c or XLPE).

In some embodiments, the tank segments 1, 5, and 6, and the coupling ring 8 may be formed of polymers filled with solids such as, but not limited to, particles or flakes of polymers or minerals, including glass, talc, carbon and graphite; chopped fibers, discontinuous fibers, short or long fibers, or continuous fibers of polymers or minerals including glass or carbon; nanocomposites; clays; or other fibers, particles, flakes or hollow microspheres. In some embodiments, any of the tank segments 1, 5 and 6 and the coupling ring 8 can be formed of metals, such as but not limited to steels, stainless steels, aluminum, or the like. In some cases the dome segments 5 and 6 may further comprise fittings or valves, including those made of metals or non-metals, including but not limited to threaded fittings, compression fittings, bulkhead fittings, quick-disconnect fittings, clip or crimp fittings, air valves, ball valves, needle valves or the like. In some cases, the dome segments 5, 6 may provide surfaces on which to make additional connections through processes including but not limited to stick welding, butt welding, spin welding, friction stir welding, ultrasonic welding, induction welding, solvent welding, RF/microwave processing, resistance-based fusion, adhesives, tie layers, or the like. These fittings, valves, or other surfaces may be connected by means known to those skilled in the art to additional system components including, but not limited to, heaters, filters, pumps, pipes, tanks, or hoses. As an example, dome 5 comprises an air valve 9 and dome 6 comprises a threaded water connection.

In certain preferred embodiments, the tank segments 1, 5, and 6, and the coupling ring 8 can be formed of polypropylene, ethylene-polypropylene copolymers, and glass particle-filled polypropylene and ethylene-polypropylene copolymers. In further preferred embodiments, the filled and unfilled he ethylene-propylene copolymers are block copolymers. The melting point and melt index of the materials forming the tank segments 1, 5, and 6, and the coupling ring 8 may be tailored to improve the assembly and processing of the tank. In certain embodiments, the outer surfaces of the tank segments 1, 5 and 6, and the coupling ring 8 may be separately or together surface modified by high energy treatments including ion implantation, plasma, corona or arc to improve adhesion to adjacent materials. The inner surfaces of the tank segments 1, 5, and 6, and the coupling ring 8 can also be modified to change properties, such as, but not limited to, the chemical resistance, permeability, or wettability by water. Treatments may include but not be limited to fluorination or the technologies employed by NBD Nano, or by metallization through chemical vapor deposition or the like. In certain preferred embodiments, the polypropylene, polypropylene copolymers, glass filled polypropylene and glass filled polypropylene copolymers are treated by a flame to improve adhesion to adjacent layers. In some preferred embodiments, the tank segments 1, 5, and 6, and the coupling ring 8 can include antimicrobials, including antifungals, antivirals, or antibiotics, or comprise silver. In other preferred embodiments, the tank segments 1, 5, and 6, and the coupling ring 8 comprise antioxidants and stabilizers.

The diaphragm 7 may be comprised of a polymer, elastomer, rubber, RTV, or thermoplastic, or multiple layers comprising the same. In certain preferred embodiments, the diaphragm 7 may be formed at least in part of butyl rubber or EPDM. In other embodiments, the diaphragm may be filled with solids such as, but not limited to, particles or flakes of polymers or minerals including glass, talc, carbon and graphite; chopped fibers, discontinuous fibers, short or long fibers, or continuous fibers of polymers or minerals including glass or carbon; nanocomposites; clays; or other fibers, particles, flakes or hollow microspheres; or woven or non-woven fabrics; to improve the thermomechanical properties or decrease permeability of gases through the membrane.

In some embodiments, where the diaphragm 7 comprises multiple layers, the layers can be bonded together, or the layers may be non-bonded. In certain embodiments, the layers include a thin higher modulus outer layer supported by a thicker, lower modulus layer. The high modulus layer is preferably selected from chemically resistant polymers, or polymers preferred for contact with potable water, such as polypropylene, polyethylene, polybutylene, or the like. The low modulus layer can be selected for different properties, such as durability, toughness, and low cost, as it is protected from contact with the potable water by the high modulus layer, and is only exposed to air or an inert gas, unless a second high modulus layer is formed on the opposite surface of the lower modulus layer.

The diaphragm 7 may also comprise features to reduce the tendency of the diaphragm to wear or fatigue in service, or protect it from abrasion or cutting by adjacent structures such as a clinch ring 3, as shown in FIG. 1C. In some cases, the diaphragm 7 may be of substantially non-uniform thickness or modulus. The non-uniform thickness or modulus may be controlled across the surface to reduce the tendency for the diaphragm 7 to rub against itself, against other structures, or to otherwise abrade or tear. The diaphragm 7 may also be substantially folded, accordion, serpentine, or wavy in shape. These shapes may allow for more compact or rigid materials to be used for manufacturing the diaphragms, while still allowing for extension in service without localized strains exceeding the limits of the materials. In other embodiments, the diaphragm 7 can be further molded or installed in the shape that it is most often present in service, to reduce the in-situ strains or abrasion, and thus extend operational life.

Prior methods of attaching the diaphragm to the expansion tank require that the equipment and processes are compatible with the rest of the tank design and methods of manufacture. In FIG. 1, the diaphragm 7 can be first joined, by its peripheral edge 4, to the coupling ring 8 by any of methods known to those skilled in the art, such as but not limited to adhesive bonding, solvent bonding, stick welding, butt welding, spin welding, friction stir welding, induction welding, RF/microwave processing, resistance-based fusion, tie layers, co-curing, or the like, with or without additional sealants. In certain preferred embodiments, the connection of the coupling ring 8 and the diaphragm peripheral edge 4 is achieved by injection molding, or overmolding, the coupling ring 8 over the diaphragm peripheral edge 4. In other preferred embodiments, the coupling ring 8 is thermoformed or hot pressed over the diaphragm peripheral edge 4. The cooling and contraction of the coupling ring 8 over the diaphragm peripheral edge 4 may further compress the diaphragm peripheral edge 4 to provide a leak proof seal. These preferred embodiments are not realistically possible to implement if the diaphragm was being connected directly to the expansion tank.

The coupling ring 8 may also be coupled with a rigid clinch ring 3 as shown in FIG. 1C. The clinch ring 3 can be formed of metal or non-metal and provides a clamping force by means known to those skilled in the art, such as but not limited to crimping; clinch ring 3, the diaphragm peripheral edge 4, and the coupling ring 8 may also comprise features or structures that improve the connection and seal to the coupling ring 8, such as lips, dimples, ridges, knobs, integral rings, including multiple rings. In certain preferred embodiments, the coupling ring 8 may have a contour that controls the radius of curvature of the diaphragm and avoids sharp edges from contacting the diaphragm. The clinch ring 3 combined with the coupling ring 8 may provide additional hoop reinforcement and further collapse resistance. The clinch ring 3 can be a continuous circumferential ring fitting within the circumferential coupling ring 8 as a single unit. Alternatively, a series of clinch members can be inserted at several locations around the circumference of the coupling ring 8, to achieve a similar effect, depending upon the separation between the individual pieces. In certain circumstances it could be easier to insert several smaller pieces than one continuous circumferential clinch ring.

In some embodiments, the tank assembly 1-10 may be further reinforced to increase the pressure carrying capabilities of the expansion tank. This reinforcement may comprise glass (including but not limited to borosilicate, e-, s, and cr-glass), basalt, quartz, carbon or other inorganic or mineral fibers. The reinforcement may also comprise organic or inorganic polymer fibers such as but not limited to polyester, nylon, polypropylene, Kevlar, Nomex, PPS. These fibers or fillers may be continuous or discontinuous fibers, chopped, non-wovens or random oriented mat, or may be in the form of fiber tapes. The reinforcing materials may be in a thermoset or thermoplastic matrix, or present without a matrix. For purposes of clarity, the reinforcement has not been shown on the outside of the tank in each Figure, but reinforcement of pressure vessels by, for example, filament winding is well known to those skilled in the art. In certain preferred embodiments, the reinforcement is a fiberglass-reinforced epoxy. In some cases, the reinforcement is a metal such as but not limited to steels, stainless steels, aluminum or the like. Preferably the reinforcing filament forms an isotensoidal wrapping, and the shape of the tank walls are fowled to be isotensoidally efficient for wrapping in an isotensoidal condition.

Critical to the operation of this diaphragm tank are the properties of the novel coupling ring 8, that seals the peripheral edge 4 of the single diaphragm 7 to the tank wall. As this coupling ring 8 is discrete, it can be manufactured independent of the tank sections 1, 5, 6 and joined to the diaphragm peripheral edge 4 prior to placing in the tank and attaching to one of the tank segments 1, 5, and 6. The attachment to the diaphragm can be accomplished by any means known to those skilled in the art, and then joining the combined coupling ring 8 and the diaphragm 7 to any of the tank segments 1, 5, or 6, also by known methods, depending upon the materials forming those items. Further, the dimensions and thermomechanical properties of the coupling ring 8 can be selected to provide substantial collapse resistance to the tank, while also allowing for ease of fabrication of the assembly of the tank with the diaphragm 8.

The coupling ring 8, the diaphragm peripheral edge 4 and the diaphragm 7 can then be joined to the tank sections 1, 5, and 6 by means known to those skilled in the art including spin welding, ultrasonic welding, RF welding, microwave welding, induction welding, friction stir welding, stick welding, resistance heating, butt welding, solvent welding, contact adhesives, chemical bonding, tie layers, or the like, with or without additional sealants. The joints between the tank segments 1, 5 and 6, and the coupling ring 8 can be independently selected from designs known to those skilled in the art, including but not limited to lap, double lap, tongue-and groove, v-groove, face, tapered, overlap, or the like.

In a preferred embodiment, the coupling ring 8 is a glass particle-filled ethylene-polypropylene copolymer that is injection molded as an independent part. The diaphragm peripheral edge 4 is inserted into the groove of the coupling ring 8 and then the coupling ring 8 is hot pressed over the diaphragm peripheral edge 4, closing the groove around the diaphragm peripheral edge 4. In another preferred embodiment, the coupling ring 8 is injection molded over 4. In a preferred embodiment, the tank segments 1, 5, and 6, and the coupling ring 8 are then joined by spin welding lap joints. Any external weld bead is then ground off. The tank is then flame treated for subsequent adhesion to a fiberglass-epoxy reinforcement.

It should be obvious to one skilled in the art that there are a number of different options to design, manufacture, and assemble a pressure tank, with the single diaphragm using the novel coupling ring 8. For example, the cylindrical tank segment 1 may be substantially short or absent so that the tank forms a substantially spherical shape. Similarly, the coupling ring 8 can be joined flush, proud or inside the tank segments 1, 5, and 6. The coupling ring 8 may also be constructed and connected to the diaphragm in a manner to minimize the strain on the diaphragm under the most prevalent position of the diaphragm in operation, or the most extreme conditions, or the conditions that would otherwise result in failure. In some tank designs, it may be beneficial to have two or more diaphragms, with one or more diaphragms connected through a single coupling ring.

In another preferred tank embodiment, for example a tank having the general shape shown in FIG. 1 or 2, the individual cylindrical and dome-shaped portions and the coupling ring of the tank are separately fabricated; the coupling ring is sealingly connected to the diaphragm peripheral edge. Before assembling the tank segments, the coupling ring with the attached diaphragm is sealingly connected to an inside surface of the cylindrical segment or to one of the dome segments. The three tank segments are then connected as described above, to form a completed tank including the diaphragm inside.

FIG. 2 is a cross section of the lower half of an expansion tank with a single diaphragm; the tank is charged with air but not. ater. For convenience, tank segments 1, 5, and 6 and diaphragm 7, whether or not shown in FIG. 2, have the same meaning as referenced for FIG. 1. For clarity, air and water fittings, as well as the upper portion of the tank are omitted, but can be similar to FIG. 1. The tank comprises a substantially cylindrical, central housing segment 1, joined to a substantially hemispherical or dome-shaped end segment 5 and to an upper hemispherical or dome-shaped end segment 6 (not shown).

In this embodiment, the peripheral edge 4 of the diaphragm 7 is connected to the lower dome section 6 by a tie-layer 18. The upper dome segment 5, not shown, could comprises an air valve, 9, which allows one area of the tank to be charged with air or gas. The lower dome segment 6 could further include a threaded connection through which water can flow into the interior volume of the tank, below the diaphragm.

In another embodiment shown in FIGS. 3 and 3B, an open metal clinch ring, 3, is coated in a thermoplastic polymer, as described above, for example ethylene polymers and copolymers. The clinch ring is then crimped on the flexible diaphragm, 7, at the peripheral edge 4. The assembled ring and diaphragm are then inserted inside the cylindrical housing, 1. The thermoplastic-coated clinch ring is then joined to the thermoplastic cylindrical housing 1, by applying induction, RF, or microwave energy which is received by the metal susceptor, causing the two polymers to heat, melt, and fuse together. In other embodiments, other heat sources, such as conduction, spin winding, or ultrasonic welding, may also be used to fuse the thermoplastics. Alternatively, a suitable tie-layer between the metallic clinch ring and the wall of the tank can be used, such as an acid-modified alkylene polymer such as one of the NUCREL ethylene-acid copolymers, e.g., NUCREL AE, a terpolymer of ethylene/methacrylic acid/acrylic acid. The tie-layer can be used to coat the metal clinch ring, and in some instances, the coating may be present only on the outer circumference of the clinch ring.

It is well known by those skilled in the art that it is extremely difficult to join dissimilar materials of construction in the expansion tank, especially flexible or elastomeric diaphragms to the rigid or semi-rigid, and chemically inert tank segments 1, 5, or 6. Differences in the chemical structures of these components make it extremely difficult to chemically react both materials with each other and form covalent bonds. Further, there may be no practical means to form covalent bonds with the chemically inert materials used for the domes and cylinders. Differences in melting point, softening points, glass transition temperatures, and crystallization temperatures can make it challenging to melt or fuse the materials together. Further, the differences in moduli of the two materials can cause significant interfacial stress when a force is applied to the joint, such as in the movement of the diaphragm during normal use. Previous attempts by those skilled in the art to bond diaphragms to the inside of expansion tanks have relied on adhesives. Since there are no covalent bonds, as the joint is exposed to moisture, temperatures, deformation, pressure changes, or thermal or mechanical cycling, “adhesive failure” often results, in which the joint cleanly separates leaving original surfaces exposed.

Unique to the expansion tank in FIG. 2 is a “tie-layer” used to provide a cohesive, covalent bond between the flexible or elastomeric diaphragm 7 and one of the rigid or semi-rigid tank segments 1, 5, or 6. This “tie layer” is a thermoplastic material that provides adhesion to two adjacent materials through melt processing or chemical reactions. Tie layers contemplated herein, that are preferred, include modified acid olefinic polymers, acrylic acid- or anhydride grafted- olefinic polymers, or those similar to but not limited to DuPont's Bynel, Nucrel, and Fusabond grades, or those described and referenced in U.S. Pat. Nos. 7,807,013 and 7,285,333. The melting point or melt index of the tie layer may be selected so that the tie-layer can be post-processed without substantially melting or flowing other non-metallics in the structure. The tie-layers contemplated and described herein, prevent adhesive failure and provide for full strength, covalent, cohesive joints.

In one embodiment, a 1-inch wide strip of 0.005-inch thick acid-modified polypropylene is placed around the inside circumference of dome 6, in the location identified in FIG. 2, adjacent to the joint with the central cylindrical segment. The diaphragm is placed inside the dome segment so that the peripheral edge is in contact circumferentially with the tie layer. Heat and pressure are applied from one or both sides of the dome wall, causing the tie-layer to melt into the dome and chemically react with the diaphragm, forming a cohesive bond not previously known to the expansion tank industry. In a preferred embodiment, the dome wall 5, 6 also reaches its melting point at the dome/tie-layer interface, during this fabrication process.

In another preferred embodiment, an anhydride-grafted PE/PP copolymer tie-layer is co-extruded on the inside of cylinder section 1, as is shown in FIG. 2B. Because the cylinder section 1 is co-melted with the tie-layer, the two materials are intimated fused. The diaphragm is placed inside the liner and heat and pressure are applied from one or both sides causing the tie-layer to chemically react with the diaphragm peripheral edge. This procedure can have the advantage of orienting the diaphragm to best support high water pressures.

The fused, covalently and cohesively bonded diaphragm may be further compressed against the wall of the tank by a rigid internal ring, or by crimping the outside wall of the tank in towards the diaphragm and ring, but that may not be necessary due to the permanent, durable nature of the novel bonding methodology.

The tank segments 1, 5, and 6, and the diaphragm 7 may be fabricated by means known to those skilled in the art, including but not limited to extrusion, injection molding, over molding, thermoforming, or the like, and may each be assembled from individual parts by means known to those skilled in the art such as but not limited to butt welding, spin welding, ultrasonic welding, RF welding, induction welding, microwave welding, induction welding, friction stir welding, stick welding, resistance heating, butt welding, solvent welding, contact adhesives, chemical bonding, or the like, with or without additional sealants.

The tank segments 1, 5, and 6, and the diaphragm 7 and the tie layer 18 may then be assembled into an expansion tank by means known to those skilled in the art, including but not limited to butt welding, spin welding, ultrasonic welding, RF welding, induction welding, microwave welding, induction welding, friction stir welding, stick welding, resistance heating, solvent welding, contact adhesives, chemical bonding, or the like, with or without additional sealants. 

What is claimed is:
 1. In an expansion tank for at least temporarily storing a pumped liquid under pressure, the expansion tank comprising a thin walled outer shell and a diaphragm located internally of the tank and secured to the inner surface of the shell of the tank to divide the internal volume of the tank into a fluid-tight section for holding a gas under pressure and a fluid-tight section for holding a liquid under pressure; the improvement comprising a diaphragm coupling ring fabricated independent of the tank segments and a diaphragm separating the inner volume of the tank into mutually fluid tight volumes, the coupling ring being sealingly, circumferentially connected between the inner circumferential surface of the tank wall and the peripheral edge of the diaphragm, wherein the completed tank was formed by first connecting the coupling ring to the diaphragm, then connecting the coupling ring to the inner wall surface of one of the tank segments, and then finally assembling the tank segments such that the diaphragm is sealed within the tank walls and the diaphragm divides the interior volume of the tank into two mutually fluid tight volumes that can be rendered fluid tight with respect to the space outside of the tank walls.
 2. The expansion tank of claim 1 wherein the coupling ring comprises a substantially level surface connected to a circumferential wall of a tank segment and the opposing face comprises a pair of lips forming a mouth for holding in a sealing connection the peripheral edge of the diaphragm.
 3. The expansion tank of claim 2, further comprising a circumferentially extending clinch ring shaped so as to fit within the mouth of the coupling ring to tightly hold the peripheral edge of the diaphragm sealingly in place.
 4. The expansion tank of claim 1 formed with a central cylindrical section and substantially hemispherically shaped end dome sections.
 5. The expansion tank of claim 1, wherein the shape of the thin walled tank is such as to be efficiently prepared for isotensoidal reinforcement by wrapping with equally stressed filaments.
 6. A method for forming the expansion tank of claim 1, the method comprising forming the central and end segments separately for forming an enclosed tank for holding a fluid and a gas in fluid-tight separation; forming a circumferential coupling ring sized to fit within and be tightly secured to the interior wall surface of a segment of the tank, the inner circumferential edge of the coupling ring formed into a mouth, and a diaphragm having a circumferentially extending peripheral edge sized to sealingly fit within the mouth of the coupling ring; securing to the inner surface of a tank segment the outer circumferential surface of the coupling ring, opposite the mouth portion holding the diaphragm edge; and combining the tank segments to form a thin-walled tank wherein the diaphragm is sealingly located internally of the tank walls to separate the inner tank volume into two mutually fluid-tight volumes for holding a liquid and a gas, respectively under pressure.
 7. The method of claim 6 wherein the diaphragm and the coupling ring are separately formed and the circumferential, peripheral edge of the diaphragm is then sealingly secured in the mouth of the coupling ring.
 8. The method of claim 6 wherein the diaphragm is separately formed and the and the coupling ring is then formed around circumferential, peripheral edge of the diaphragm so as to sealingly secure the circumferential, peripheral edge of the diaphragm in the mouth of the coupling ring.
 9. In an expansion tank for at least temporarily storing a pumped liquid under pressure, the expansion tank comprising a thin walled outer shell and a diaphragm located internally of the tank and secured to the inner surface of the shell of the tank to divide the internal volume of the tank into a fluid-tight section for holding a gas under pressure and a fluid-tight section for holding a liquid under pressure; the improvement comprising sealably securing the peripheral edge of the diaphragm to an internal, circumferential wall of a segment of the tank utilizing a tie layer that is mutually compatible with the material from which the inner circumferential surfaces of the tank segments are fabricated and with the material forming at least an outer surface of the peripheral edge of a diaphragm, the diaphragm being sealed in place so as to separate the inner volume of the tank into mutually fluid tight volumes, wherein the completed tank was formed by first connecting the diaphragm, to the inner wall surface of one of the tank segments utilizing the tie-layer, and then finally assembling the tank segments such that the diaphragm is sealed within the tank walls and the diaphragm divides the interior volume of the tank into two mutually fluid tight volumes that can be rendered fluid tight with respect to the space outside of the tank walls.
 10. The expansion tank of claim 9 wherein the tie-layer is placed between the peripheral edge of a diaphragm and the wall of a tank segment such that the material compatible with the peripheral edge of the diaphragm is in contact with the diaphragm and the opposing face compatible with the tank segment inner wall surface is in contact with the wall.
 11. The expansion tank of claim 9 formed with a central cylindrical section and a substantially hemispherically shaped end dome section.
 12. The expansion tank of claim 9, wherein the shape of the thin walled tank is such as to be efficiently prepared for isotensoidal reinforcement by wrapping with equally stressed filaments.
 13. The expansion tank of claim 9, wherein the tie layer is selected from the group consisting of acid-modified alkylene polymer, a NUCREL ethylene-acid copolymers, a terpolymer of ethylene/methacrylic acid/acrylic acid, modified acid olefinic polymers, acrylic acid- or anhydride grafted- olefinic polymers, DuPont's Bynel, Nucrel, and Fusabond grades, anhydride-grafted PE/PP copolymers,
 14. The expansion tank of claim 9, wherein the tie layer comprises two mutually compatible layers of material one of which is compatible with the material from which the inner circumferential surfaces of the tank segments are fabricated and the other of which is compatible with the material forming at least an outer surface of the peripheral edge of a diaphragm, or with the outer surface of a clinch ring gripping the peripheral edge of a diaphragm. 